Unit 3: Exchange & Transport
3 A Exchange Surfaces & Gas Exchange
Objectives
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Need for specialised exchange surfaces in
multicellular organisms – relate to surface
area to volume ratio (SA:Vol)
Features of exchange surfaces
Examples of exchange surfaces (lungs and
alveoli)
Adaptations of the lung for exchange – CO2
and O2
Ventilation and the role of lung tissues –
mechanism of breathing
Measuring lung capacity – spirometer;
Interpretation of data
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All living cells need to obtain certain vital substances from the external environment in which
they live, and need to remove (excrete) harmful waste products of metabolism into the external
environment, in order to stay alive
Need to obtain from
external environment
Nutrients
• Glucose (as a source of energy)
• Proteins (for growth + repair)
• Fats (for membrane structure; energy
store; myelin sheath in neurones)
• Water (solvent; reactant)
• Minerals (for structure; water potential)
• Vitamins (for metabolism)
Need to remove into
external environment
Waste
• CO2 (from respiration)
• O2 (from photosynthesis)
• Urea (from amino acids)
• Ammonia
• Excess heat
• O2 – for aerobic respiration
• CO2 – for photosynthesis (in plants)
Cell
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Single-celled (e.g. amoeba, bacteria, protozoa) and small organisms (e.g. flatworms) exchange
materials across their cell membranes by DIFFUSION
Diffusion - movement of molecules from a region of high concentration
to a region of lower concentration
They have large surface area to volume ratio – the exchange surface is adequate to exchange
materials by diffusion
Materials exchanged rapidly between external environment (water) by
diffusion – external environment is in direct contact with the cell
– short diffusion distance (thickness of cell membrane)
Size of cube
SA =
Volume =
SA:Volume ratio =
1 cm
6 cm2 (1x1x6)
1 cm3 (1x1x1)
6/1=6
Doubling of size causes
2 cm
24 cm2 (2x2x6)
8 cm3 (2x2x2)
24 / 8 = 3
SA to increase by 4 times
Volume to increase by 8 times
SA:Vol ratio to be halved
Smaller organisms have a higher surface area to volume ratio
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https://video.nationalgeographic.com/video
/til/161021-sciex-til-anusha-shankarhummingbirds?source=searchvideo
The Hummingbird
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Larger multicellular organisms have lower SA : Vol ratios – they cannot rely on diffusion alone
to exchange materials
Increase in SA is not proportional to increase in volume as organisms get larger – volume
increases much more than SA.
A larger organism has a lower amount of surface in contact with
the external environment in relation to its volume.
❑ Larger organisms have a greater demand for materials and produce greater
amounts of toxic waste and heat, due to the large number of cells
undergoing metabolism.
❑ Waste needs to be removed rapidly. Heat needs to be dissipated to prevent
overheating – to prevent denaturation of enzymes
❑ Cells are distant from the external environment – materials need to travel
longer distances and cross numerous barriers (membranes; fluid)
Materials cannot be exchanged fast enough to maintain life
❑ Therefore, to meet the greater demands, larger multicellular organisms need
efficient and specialised Exchange surfaces - intestine for absorption of digested nutrients;
alveoli in lungs for gas exchange, and,
Transport systems – circulatory system
Some exchange surfaces may use the following to increase the efficiency of exchange further –
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active transport (e.g. mineral uptake in root hair cells); facilitated diffusion ; bulk transport
Features of efficient exchange surfaces
Large surface area – large exchange surface to allow more molecules to pass through –
usually achieved by structural adaptations – e.g. numerous spherical alveoli in lungs ; folding
of walls and membranes - e.g. villi and microvilli in small intestine
Thin – usually single cell thick - reduce diffusion distance between source and destination -e.g.
wall of alveoli is composed of a single layer of squamous epithelium)
Steep concentration gradient - constant supply of materials to be exchanged on one side (the
supply side) to keep the concentration high (C2) and constant removal on the other side (the
demand site) to keep the concentration low (C1) – in order to maintain a steep concentration
(diffusion) gradient (C2 – C1) across the exchange surface –to ensure rapid diffusion - e.g. by
ventilation (breathing) and circulation (blood circulatory system)
Surface area X (C2 – C1)
Rate of diffusion
=
Thickness
The surface area and thickness are fixed to certain limits – the concentration gradient (C2 – C1)
is the rate-limiting factor in exchange by diffusion across exchange surfaces
Some exchange surfaces
Small intestine - villi and microvilli – increase surface area for absorption of soluble
nutrients – removed and transported to cells by the circulatory system
Root hairs of plants – large surface area for absorption of H2O and minerals
Lungs (alveoli) – large surface area for exchange of O2 and CO2
Hyphae (fungi) – large surface area for absorption of nutrients
The Need for Ventilation and Gas Exchange
Cell respiration provides the energy (ATP) requirements of the organism
❑
It requires a constant supply of O2 and produces CO2 (waste – that needs to be
excreted)
❑
The obtaining of O2 and removal of CO2 is achieved by the respiratory system
through the processes of ventilation and gas exchange
The respiratory system links the blood circulatory system with the atmosphere
The structural components of the respiratory system function to move atmospheric air into
and out of the lungs (alveoli) by the rhythmic physical process of ventilation
Ventilation
Rhythmic physical process of moving atmospheric air into and out of the
lungs (the organs of gas exchange) in order to supply O2 and remove CO2
Gas exchange
Gas exchange occurs between the air in the alveoli (the specialised
exchange surfaces in the lungs), and blood in the lung (pulmonary)
capillaries, which surround the alveoli.
❑Blood rich in CO2 (from cell respiration) is brought to the alveoli from tissues – CO2
diffuses into the alveoli down a concentration gradient; O2 (from atmospheric air in the
alveoli) diffuses into the blood from the alveoli, down a concentration gradient
❑The large number of thin walled tubular capillaries provide a rich blood supply and a
large surface area for gas exchange between the blood and atmospheric air
❑The large number of spherical alveoli provides a large surface area for gas exchange
(approximately 80 -100 m ) between the atmospheric air and blood
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Pulmonary vein
Oxygenated blood
▪
Nitrogen does not cross the
alveolar wall – nitrogen is not
used or produced in the body
▪
O2 is required for cell respiration
to release energy from glucose
▪
CO2 (waste) is produced in cell
respiration – needs to be
removed (excreted)
▪
Water vapour varies – depending
on humidity of atmospheric air
▪
Exhaled air is always more
humid than inhaled air
Pulmonary artery
Deoxygenated blood
Vena cava
Deoxygenated blood
Aorta
Oxygenated blood
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Respiratory System
Rib
Internal & external
intercostal muscles
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Lungs (2)
The lungs are the organs of gas exchange and
are composed of a number of different tissue
types – e.g. elastic, connective, epithelial,
nervous, muscular - working together to
achieve particular functions – i.e. ventilation
(breathing) and gas exchange between the air
and blood
• CO2 from the blood into atmospheric air
• O2 from atmospheric air into blood
Air can pass into and out of the lungs, through
the respiratory tree (airways), which consists
of the nose, the trachea (windpipe), bronchi
and bronchioles.
The bronchioles end in spherical structures,
termed the alveoli. Gas exchange between
the air in the alveoli and blood (in the lung
capillaries) takes place across the thin walls
of the alveoli
The lungs are a large pair of inflatable
structures (organs) located in the chest
(thoracic cavity)
Pleural cavity contains pleural fluid
▪ Reduces friction during ventilation
▪ Defence
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Respiratory Tree
Atmospheric air
Direction of air flow
during inhalation
Larynx (“voice box”)
Trachea
Bronchus (2)
Bronchioles
Direction of air flow
during exhalation
Alveoli (“air sacs”)
Site of gas exchange
Gases exchanged by diffusion down their concentration gradients
O2 from alveoli to blood in alveolar capillaries
CO2 from blood into alveoli (for excretion)
Capillaries (blood)
Red blood cells transport absorbed O2 from lungs to body tissues
CO2 transported from tissues to lungs for excretion
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Adaptations of Respiratory System for Gas Exchange
❑
Cleans, warms, and moistens the air entering the respiratory tract during inhalation –
achieved by hairs, mucus, and blood capillaries (warm blood) in the nasal cavity
and cilia in the trachea
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A large number of spherical alveoli (600 million) 100-300 um across – giving a large
surface area of approx.80-100 m2 for rapid gas exchange between the blood and
atmosphere; alveolar wall is one cell thick (squamous epithelium) – short diffusion path
between alveoli and blood in the pulmonary capillaries
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Numerous and tubular capillaries in close contact with alveoli - wall also made of squamous
epithelium and one cell thick; total diffusion distance is less than 1 um
❑
Capillaries are narrow – red blood cells are squeezed against the capillary wall –
making contact with the wall to reduce diffusion distance. The blood cells are also slowed down –
increasing the time for exchange
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Constant ventilation - replacing the air in the alveoli to maintain a concentration
gradient between the alveolar air and the blood
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Alveoli are surrounded by a large number of capillaries – a rich blood supply, replaces
the blood constantly, to maintain a concentration gradient between the blood and alveolar air.
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Haemoglobin (Hb) in red blood cells carries oxygen and has a high affinity for oxygen – helps
in maintaining a steep concentration gradient for oxygen
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Moist exchange surface – allows gases to dissolve
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Water in the alveoli contains a surfactant (phospholipid) – reduces surface tension –
prevents collapse of alveoli
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Alveoli contain phagocytic cells - for defence
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O2 from the atmospheric air entering the alveoli
enters the blood by diffusion across the alveolar wall
down its concentration gradient
Total barrier to diffusion is less than 1 um
CO2 from the blood enters the alveoli by diffusion
across the alveolar wall down its concentration
gradient
Oxygenated blood to
heart via pulmonary vein
Low CO2 concentration
Surfactant produced by alveoli –
reduces cohesive surface tension
forces between H2O molecules in
film of H2O
ST forces may cause collapse of
alveoli during exhalation – prevented
by surfactant
Diffusion gradient maintained by
rhythmic ventilation (replaces air in
alveoli) and a rich blood supply high [O2] on supply side (alveoli);
low [O2] on demand side
(capillaries) – vice versa for CO2
Squamous (flattened)
epithelium
Plasma – carries 85% of CO2
as hydrogen carbonate ions,
5% dissolved in plasma, and
10% in combination with
haemoglobin
O2
CO2
Deoxygenated blood from
heart via pulmonary artery
High CO2 concentration
Capillaries
Close contact with alveolar wall
1 cell thick – short diffusion distance
Narrow – space enough for passage of
single RBC – allows close contact with
capillary wall
Large surface area – numerous and tubular
Alveoli – spherical and numerous (600
million); large surface area (c.70 m2)
Film of H2O
– dissolves gases
- evaporates
Alveolus wall
Capillary wall
Each 1 cell
thick
RBC’s – contain haemoglobin carries O2 as oxyhaemoglobin
Ventilation – movement of air into and out of lungs
The ribcage, intercostal muscles and diaphragm all work together to move air into and out
of the lungs, where gas exchange occurs across the thin (single-celled) walls of the alveoli
Ventilation is a physical process, relying on the principle of Boyle’s Law – which state
“Pressure is inversely proportional to volume”
The mechanism can be illustrated using a bell jar model of the respiratory system – however,
the model does not illustrate involvement of the rib cage and the intercostal muscles in
ventilation
Breathing out
(expiration /
exhalation)
Internal intecostals contract
in forced expiration
Breathing in
(inspiration /
inhalation)
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INSPIRATION
EXPIRATION
Atmospheric pressure = 760 mmHg
External intercostals
contract raising the rib cage
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Diaphragm & external intercostals contract
Rib cage raised (upwards and outwards)
Diaphragm lowered (becomes flatter)
Volume of chest cavity increases
Pressure in chest cavity drops to below
atmospheric pressure to 758 mmHg
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Air moves into lungs from atmosphere
Active process
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External intercostals relax
lowering the rib cage
Diaphragm & external intercostals relax
Rib cage lowered
Diaphragm raised (dome shape) due to push
from abdominal organs
Volume of chest cavity decreases
Pressure in chest cavity increases to
above atmospheric pressure to763 mmHg
Air forced out of lungs into atmosphere
Aided by elastic recoil and abdominal organs
Passive process
Structural components of the respiratory system
Trachea
Smooth muscle
Contracts and relax to allow diameter of airways to be controlled;
During exercise the muscle relaxes – making the airways wider –
reduces resistance to air flow – aids ventilation
Muscles contracts to narrow the airways when challenged with
foreign material (e.g. pollen) to protect airways and alveoli
Elastic fibres
Stretch to allow expansion during inhalation and recoil during
exhalation; prevent over expansion
C-shaped rings of cartilage
Provide structural support
Prevent collapse of airway during inhalation
Allows flexibility during movement without narrowing of airways
Allows oesophagus to expand during swallowing
Inside surface of trachea – epithelial lining
Goblet (mucus) cells
Secrete mucus – traps particles (e.g. dust, pollen, bacteria) –
reduce risk of infection & inflammation
Ciliated epithelium
Cilia beat in a synchronised pattern to move (waft) mucus (with
particles) towards throat – to be swallowed (stomach acid kills
bacteria) or expectorated; prevents infection
Cells contain numerous mitochondria (energy for ciliary movement)
Loose tissue
Inside surface of cartilage – glandular tissue, connective tissue,
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elastic fibres, smooth muscle and blood vessels
Bronchi
▪ Two narrower branches arising from the trachea – the right is more vertical and wider than
the left – protective function – allows material entering the airways by accident to be
directed into the right side, keeping the left functional
Similar in structure to trachea – cartilage provides mechanical strength, and prevents collapse
▪ Cartilage is less regular
Bronchioles
▪ Branches arising from the bronchi - much narrower than bronchi
▪ Some cartilage in larger ones – none in smaller ones
▪ Mainly smooth muscle and elastic fibres – terminal bronchioles have clusters of alveoli at their
ends, where gas exchange takes place
Smooth muscle
▪ Contracts involuntarily to constrict the airways – narrows lumen – most obvious in bronchioles;
restricts airflow to and from the alveoli
Defensive to prevent entry of foreign material and agents triggering contraction (e.g. in
asthma). – leads to difficulty in breathing – drugs termed bronchodilators may be used to
alleviate symptoms
Elastic fibres
▪ Smooth muscle cannot reverse the effect of narrowing of the airways. Elastic fibres in the
loose tissue are deformed when smooth muscle contracts. As the muscle relaxes, the
elastic fibres recoil to their original size and shape – helping to dilate the airways.
▪ Elastic fibres in the lungs aid recoil of lung tissue during exhalation.
Phagocytic cells
▪ Mostly in alveoli - defence
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Cross section of bronchiole
Alveolus wall
▪ Thin - single cell thick
▪ Squamous epithelium
▪ Reduces diffusion distance
Bronchiole wall
▪ Ciliated epithelium (cilia
move mucus upwards)
▪ Goblet cells (secrete
mucus)
Blood capillary
▪ Close to alveoli
▪ Thin - single cell thick
▪ Squamous epithelium
▪ Reduces diffusion distance
Pulmonary vein
▪ Carries oxygenated blood to heart
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Bronchiole (a) and trachea(b) in transverse section
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Ciliated epithelium
▪
Composed of cells that possess cilia
and mucus secreting goblet cells
and glands
▪
The exposed part of the cell surface
(luminal surface) is covered with cilia
Cilia are hair like projections of the
cell membrane – similar in structure
to flagella, but much smaller
▪
Found in the trachea, bronchi,
and in larger and medium-sized
bronchioles
▪
Cells can be columnar or cuboidal
▪
Move fluid or mucus (with trapped
material) upwards in the trachea by the mucociliary escalator; able to
filter out particles between 2-10
microns (um) in diameter
Nicotine in cigarette smoke
paralyses cilia; tar also damages cilia
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Part of lung
Cartilage
Smooth
muscle
Yes
Elastic fibres
Goblet cells
Epithelium
Trachea
Large Cshaped rings
Yes
Yes
Ciliated
Bronchi
Smaller pieces
Yes
Yes
Yes
Ciliated
Larger
bronchiole
Some
Yes
Yes
Yes
Ciliated
Smaller
bronchiole
None
Yes
Yes
None
Ciliated
Smallest
(terminal)
bronchiole
Alveoli
None
Yes
Yes
None
No cilia
Yes
Recoil after
deformation to
dilate (widen)
the airway
None
Secrete mucus
– traps tiny
particles (e.g.
pollen;
bacteria) from
the inhaled air
No cilia
Cilia move in a
synchronised
pattern to waft
the mucus up
the airway to
the back of the
throat, to be
swallowed.
The acidity of
the stomach
kills any
bacteria
None
Structural
Supports
trachea and
bronchi –
holding them
open
None
Contracts to
constrict the
airways –
restricting the
flow of air
In the lungs –
recoil to
reduce lung
volume in
expiration
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Measuring Lung Volumes - Spirometry
Lung volumes can be measured using a spirometer
and recorded as a spirometer trace (spirogram).
Used to assess lung function under various conditions)
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to evaluate respiratory impairment
to aid diagnosis by identification of particular disease
to assess changes with time and/or treatment
to help assess fitness for anaesthesia and surgery
Larger volumes
Smaller volumes
males
Females
taller people
shorter people
non-smokers
heavy smokers
athletes
non-athletes
people living at
high altitudes
people living at low
altitudes
▪
A chart recorder is used to produce
a graph (spirogram)
▪
Soda lime (sodium hydroxide)–
absorbs CO2 from exhaled air
▪
The chamber does not rise to the
same height with each breath –
since, O2 is consumed and the CO2
produced is absorbed by the soda
lime
▪
Depending on the design, in some
respirometers, the float rises during
inspiration and falls during
expiration – the inspiration peak is
longer than the expiration peak
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Spirogram
Inspiratory reserve volume
– larger than expiratory reserve volume
Expiratory reserve volume
– less than inspiratory reserve volume
Normal breathing rate = 12-18 breaths per minute
One breath = Inspiration + Expiration
Two factors determine ventilation of the lungs – RATE and DEPTH of breathing
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Volume
Definition
Tidal Volume (TV)
0.5 L
▪ Volume of air moved into and out of lungs with each breath
at
rest.
▪ Provides body with enough O2 for its resting needs while
removing enough CO2 to maintain a safe level
Vital Capacity (VC)
5 L (male)
4.5 L (female)
▪ Largest volume of air that can be moved into and out of the
lungs in any one breath (IRV + TV + ERV); varies (age, sex,
gender, disease).
▪ Regular exercise increases VC
Inspiratory Reserve
Volume (IRV)
3L
▪ How much more air that can be inhaled over and above the
normal TV when taking a deep breath.
▪ Utilised when exercising, defecating, playing musical
instruments, etc
Expiratory Reserve
Volume (ERV)
1.2 L
▪ How much more air that can be exhaled over and above the
amount that is expired in a normal TV breath
Residual Volume (RV)
1.5 L
Dead Volume (DV)
▪ Volume of air remaining in the lungs, even after forced
exhalation
▪ Volume of air in the bronchioles, bronchi and trachea – does
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not take part in gas exchange
Using a spirometer
1.Subject - relaxed and allowed to establish a breathing pattern
2.A sterile mouthpiece is placed into the mouth; and a nose clip worn– nose clip ensures
all air breathed in comes from the chamber and also prevents entry or exit of air through the
nose
3.Subject breathes normally and the kymograph started; a data logger may be used
4.A spirometer trace (spirogram) is obtained - this is the TIDAL VOLUME
5.After a few minutes of relaxed breathing, the subject performs maximum expiration (by
forcing all the air out of the lungs) and then performs a maximum inspiration, before
returning to a normal breathing pattern.
This gives a record the EXPIRATORY RESERVE VOLUME, INSPIRATORY
RESERVE VOLUME, and VITAL CAPACITY
Precautions
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Ensure subject is not on medication related to respiratory illness
Ensure subject is not suffering from respiratory problems (e.g. asthma)
Ensure fresh supply of soda lime for each investigation
Ensure equipment is sterile and disinfect mouthpiece
Use medical grade O2 and ensure adequate amount of O2 is available
Ensure equipment is functioning correctly
Maintain constant temperature – temperature affects volume of gases
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Calculations involving lung volumes
Respiratory Minute Volume (RMV)
The RMV (or minute volume) is the volume of air inhaled or exhaled from a person’s lungs in
one minute – it is also known as pulmonary ventilation (PV)
It is the product of the tidal volume and the breathing rate
Normal - 5-8 dm3 per minute
Example
Abnormal – e.g. in asthma – up to 14 dm3 per minute
Tidal volume = 0.5 dm3
Breathing rate = 12 breaths /minute
RMV = TV x breathing rate = 0.5 x 12 = 6 dm3 /minute
Breathing rate from a spirogram
1Count the number of peaks (tidal volumes)
in a given time
3 peaks in first 20 seconds
2Calculate the number of peaks in one minute
No of peaks in 1 minute
= 3 x 60 / 20 = 9 peaks per minute
Breathing rate = 9 breaths per minute
(bpm)
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O2 uptake per minute from spirogram
Example
If
x (decrease in volume) = 0.8 dm3
y (time) = 130 – 20 = 110 sec
In 110 s, 0.8 dm3 of O2 is used up
x / y = 0.8 / 110 x 60 =
0.44 dm3of O2 used per minute
Breathing rate
= 22 / 110 = 0.2 breaths/second
= 0.2 x 60
= 12 breaths/ minute
❑
The volume of air inspired is the same as the volume expired.
❑
Soda lime absorbs the CO2 exhaled – causing the total volume of air to go down.
❑
This total reduction is equal to the volume of O2 breathed in – allowing the estimation of
O2 use under different conditions.
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Exercise and pulmonary ventilation
The interdependence of the circulatory and the respiratory system in supplying O2 to tissues
for respiration and the removal of CO2 (waste) for excretion requires an increase in pulmonary
ventilation during exercise – to ensure that the demand for extra O2 and the removal of the
excess CO2 during exercise are met
Effects of exercise on PV
Exercise causes an increase in PV (as well as
cardiac output)
This is to deliver greater amounts of O2 faster to the
blood and therefore the respiring muscle cells and to
remove CO2, lactic acid, and excess heat
Chemoreceptors in the medulla oblongata, the
carotid and aortic bodies detect the decrease in pH
during exercise due to the increase in CO2 produced
Signals are sent to the respiratory centre in the
medulla to increase the frequency of impulses to the
diaphragm and intercostal muscles to increase the
breathing rate (breaths per minute) and depth (tidal
volume) of breathing to increase PV
This allows gaseous exchange to speed up –
causing CO2 levels to drop and the demand for extra
O2 to be met
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